Method of treating impulse turbine buckets



y 1957 E. R. PARKER ETALV 2,801,193

METHOD OF TREATING IMPULSE TURBINE BUCKETS iled May 14, 1954 Fig.2

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id States Our invention relates generally to a manner of treating bulky metal parts subject to fatigue failure. While it is by no means limited thereto, the method is particularly concerned with improvement in the performance characteristics of impulse turbine buckets. These buckets are in practice arranged around the periphery of a rotary water wheel and involve a relatively thin leading edge which enters the jet or nozzle stream of water, traverses the stream, and emerges therefrom during the revolution of the wheel. The velocity of the water striking the bucket edge and the remainder of the bucket is relatively high. It has been found in practice that the thin leading edge or entrance lip portion, particularly, of the bucket is subject to fatigue failure. Since efficiency of the wheel as a whole is a large consideration, it is not feasible to make the bucket lip massive and stiff for to do so reduces the efficiency severely. There is consequently a. need for prolonging the bucket lip life or of reducing or eliminating fatigue failure in buckets of an efficient design.

There are various means utilized in industry for improving the characteristics of various products to resist fatigue failure, such as shot peening, rolling and nitriding. These treatments, however, are not well adapted to an object as bulky as the customary impulse turbine bucket,

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It is therefore an object of our invention to provide a treatment method eifective to reduce or even entirely to eliminate fatigue failure in turbine buckets and particularly in the entrance lip portion thereof.

Another object of our invention is to provide a method which-is sufficiently simple to utilize in the field for treating impulse turbine buckets.

Another object of our invention is to provide a method of treating impulse turbine buckets applicable to new buckets as well as to those which have already been placed in service. i l

A still further object of the invention is to provide a method of treating impulse turbine buckets which have been constructed by fabrication or which have welds in them, especially in the entrance lip area.

Another object of our invention is in general to provide an improved method for treating impulse turbine buckets against failure.

Other objects, together with the foregoing, are attained in the practice of our method as set forth in the accompanying description which is to be read in connection with the accompanying drawing, in which:

Figure 1 is a bottom plan view of a typical impulse turbine bucket arranged for treatment in accordance with our method.

Figure 2 is a cross-section, the plane of which is indicated by the line 22 of Figure l.

The practice of the method of our invention is Well exemplified in the case of an impulse turbine bucket although itis by no means limited solely to that environatnt nor are they well adapted to an object of such an irregular of water playing upon the zone for that purpose.

to receive a through bolt or similar fastening means, not shown. The lugs 6 and 7 are extended to provide ribs 11 and 12 merging with the bucket body 13. This struc ture is symmetrical about a central, longitudinal plane perpendicular to the axis of the apertures 8 and 9 and includes a pair of merging bowls 14 and 16 of substantial thickness throughout but adjacent their leading central portion they become considerably thinner as they merge into an entrance lip 18.

The lip is likewise symmetrical about a central plane and is distinguished by a projecting tongue 19 formed as part of a central splitter 21 characteristic of a twin bowl cold bucket. The entire periphery of the entrance lip, distinguished by shading 22, is especially susceptible to fatigue failure evidenced by the eventual appearance of minute cracks principally extending inwardly from the edge of the bucket. These cracks can enlarge until failure of the entire bucket and perhaps of the entire wheel transpires.

it is customary for each bucket, including the lugs 6 and 7, to be cast of one piece of metal, usually steel but sometimes bronze or a related material. It is often necessary, particularly in cast bucket bodies of substantial size, to eliminate casting imperfections by removing faulty material and filling the resulting void with weld metal. The resulting weld is a satisfactory repair providing the weld is sound but often it is difficult to determine even with extraordinary inspection procedures, whether or not an appropriately homogeneous weld has been made. In some instances, the bucket body is not cast but is fabricated of metal shapes secured together by welding. Whether the bucket is primarily cast or fabricated it is not unusual to find a bucket body with welding included in it. This welding, if not adequately done, itself fails under repetitive stress and may cause failure of the entire bucket and perhaps of the entire wheel.

To overcome these difficulties, we have provided a method of treating an impulse turbine bucket to reduce substantially the occurrence of cracks or splits due to fatigue failure and our method is also effective as a check or test to determine the sufiicicncy and homogeneity of welded portions of the bucket.

In carrying out our method we take a bucket 13, either when it has been newly manufactured or after it has been in service for a time and whether it is cast or fabricated and whether or not it includes welds, and treat it thermally. In doing so, we preferably position a bucket substantially as shown in Figure l with the'convex portion of the bucket upwardly and with the bucket rim in approximately a horizontal or slightly inclined position. We then erect a temporary barrier or dam 23 across or transversely of the bucket body, at least between the ribs 11 and 12 and a short distance toward the fastening openings 8 and'9 from the entrance lip area 22. The precise distance between the entrance lip edge and the barrier 23 is not critical but in the usual size of buckets ,is from about one and one-half inches to two and onehalf inches. The barrier 23 is not securely attached to the bucket but is merely put in place by any convenient means. The barrier itself can be any material usually available in the shop or in the field and can be a plate of relatively light metal, a sheet of asbestos, or any like material. v

In accordance with the invention, the approximate area 24 (partly indicated by the shading 22) is considered as a zone immediately adjacent the entrance lip of the bucket and is maintained at room temperature or somewhat cooler. In any event, the temperature in the zone 24 is not permitted to rise to any substantial extent. For that reason we provide a coolant in surface contact with the zone 24 of the bucket preferably utilizing a stream One of the functions of the barrier 23 is to serve as a dam or check to confine the coolant to one side, that is, to the lip side, of the barrier. If necessary, some sealing means such as clay can temporarily be put around the meeting edges of the barrier and the bucket.

On the other side of the barrier 23 that is, on the side thereof away from the entrance lip 22, and in the central zone of the bucket body 13 we apply heat in order to elevate the temperature. This is preferably accomplished by means of a gas torch flame played against the surface of the bucket body on one side of the barrier 23 only Without encroachment upon the entrance lip zone 24. Thus, the barrier 23 serves also as a heat dam to prevent flame impingement upon the lip zone of the bucket.

The heat is added to the region of the bucket body behind the barrier 23 for such a time and to such an extent as to raise the temperature of that portion, at least, of the bucket body to about 350 degrees Fahrenheit.

This temperature varies with the particular material of the bucket but for steel buckets is about of the value glven. In practice, we preferably do not stop at 350 degrees but actually run the temperature up to about 400 degrees Fahrenheit in order to make sure that the yield point of the bucket material has been exceeded. The

effect of the localized heating is to expand the bucket body in the heated zone but since the lip zone is not heated, it does not expand thermally. However, the elongation transversely of the adjacent, expanded heated zone puts a sufiicient force on the contiguous material of the bucket so that the bucket material in the entrance 11p zone is subjected to a substantial transverse stress in tension resulting in the material passing its yield point and taking a permanent set or elongation.

When this has been accomplished, we remove the torch or other source of heat from the bucket body zone and permlt that zone to cool. As the bucket body returns toward room temperature, it contracts and in doing so it shrinks transversely and imposes a transverse compressive stress upon the entrance lip zone. The material 1n the zone 24, even to the lip edge 22, is put in compresslon and is maintained in compression when the temperature of the entire bucket body is uniform.

We have determined that the entrance lip material under compression is far less susceptible to fatigue failure than the usual unstressed entrance lip material. In fact, the treatment is sufiiciently effective so that fatigue failures or fatigue cracks in the entrance lip zone of impulse turbine buckets have not been observed since the described method has been employed upon them.

While the heated bucket body is cooling down after the withdrawal of the heating source, Water continues to. be played upon the entrance lip zone to make sure that no temperature risein that area occurs. Finally, when both portions have reached substantially the same room temperature, the playing of Water upon the entrance lip zone 24 is discontinued, the temporary barrier 23 is removed, and the bucket, without more, is ready for attachment to its wheel and for service.

In those instances wherein the bucket body has been provided with welds, either as initial or subsequent repairs or as a means of fabrication, the present method is effective to show up flaws in such welds and, in fact, will also show up flaws in the bucket material as well. The transverse elongation of the entrance lip portion during the initial heating of the body portion of the bucket is elfective to place the weld and adjacent material under a considerable stress in tension, enough to cause Weld failure if the Weld is weak. In fact, to increase the temperature gradient and so increase the forces involved, Dry Ice is sometimes used for cooling the entrance lip zone below room temperature While the body portion is being heated. Consequently, a bucket being so treated can simultaneously be checked for material and weld soundless in the critical area of the entrance lip.

If the material and the weld withstand the localized elongation, it is safe to assume that the weld and the material are sound and will not in future cause trouble.

In general, in accordance with our invention, we have provided a method of treating impulse turbine buckets Without any change in the present bucket design or fabrication which can be applied in the shop or in the field and is effective upon various buckets substantially to eliminate cracks or other failures due to fatigue.

What is claimed is:

1. A method of treating impulse turbine buckets comprising erecting a barrier for liquid across a bucket and spaced from the entrance lip thereof, subjecting the material of said bucket on one side of said barrier between said barrier and said entrance, lip to a relatively cool temperature, simultaneously subjecting the material of said bucket on the other side of said barrier to a temperature in excess of 350 degrees Fahrenheit, cooling said material on said other side to room temperature while maintaining said material on said one side relatively cool, and removing said barrier.

2. A method of treating impulse turbine buckets comprising maintaining the material of said bucket in a zone immediately adjacent the entrance lip of the bucket relatively cool throughout the process, heating the material of said bucket next to said zone to a temperature at which the material within said zone is stretched beyond its yield point, and then cooling the material of said bucket next to said zone substantially to room temperature.

3. A method of treating impulse turbine buckets comprising maintaining the material of said bucket immediately adjacent the entrance lip of the bucket relatively cool while heating and then cooling the surrounding bucket material sufficiently to leave a compressive stress in the material of said bucket immediately adjacent said entrance lip.

4. A method of treating impulse turbine buckets comprising heating an interior portion only of a cold bucket sufficiently to stress an adjacent edge portion beyond the yield point of said cold edge portion, and then cooling said interior portion sufliciently to stress said cold edge portion in compression.

5. A method of treating impulse turbine buckets comprising heating an interior portion only of a bucket above 350 degrees Fahrenheit, then cooling said interior portion to room temperature, and during all of said heating and cooling keeping an adjacent edge portion substantially at room temperature.

6. A method of treating impulse turbine buckets comprising heat treating a body portion only of sid bucket sufiiciently to stretch permanently an unheat-treated adjacent edge portion of said bucket.

7. A method of treating impulse turbine buckets comprising contacting the edge portion only of said bucket with Water substantially at room temperature while heating an adjacent portion of said bucket body above 350 degrees Fahrenheit and while subsequently cooling said adjacent portion to room temperature.

8. A method of treating impulse turbine buckets comprising heating an interior portion of said bucket sufficiently to stress an adjacent edge portion of said bucket beyond its yield point, then cooling said interior portion, and during said heating and cooling protecting said edge portion from a substantial rise in temperature by a heat barrier disposed between said interior portion and said edge portion and by putting water on said edge portion.

References Cited in the file of this patent UNITED STATES PATENTS 2,209,674 Burish July 30, 1940 2,227,476 Williams Jan. 7, 1941 2,315,558 Somes Apr. 6, 1943 2,364,235 Muros. Dec. 5, 1944 2,682,936 Almen July 6, 1954 

1. A METHOD OF TREATING IMPULSE TRUBINE BUCKETS COMPRISING ERECTING A BARRIER FOR LIQUID ACROSS A BUCKET AND SPACED FROM THE ENTRANCE LIP THEREOF, SUBJECTING THE MATERIAL OF SAID BUCKET ON ONE SIDE OF SAID BARRIER BETWEEN SAID BARRIER AND SAID ENTRANCE LIP TO A RELATIVELY COOL TEMPERATURE, SIMULTANEOUSLY SUBJECTING THE MATERIAL OF SAID BUCKET ON THE OTHER SIDE OF SAID BARRIER TO A TEMPERATURE IN EX CESS OF 350 DEGREES FAHRENHEIT, COOLING SAID MATERIAL ON SAID OTHER SIDE TO ROOM TEMPERATURE WHILE MAINTAINING SAID MATERIAL ON SAID ONE SIDE RELATIVELY COOL, AND REMOVING SAID BARRIER. 